Nitroso compounds show great versatility as carcinogenic agents. Knowledge about a protection by diet from the carcinogenic action of nitroso compounds is of prevalent interest in relation to carcinogenesis in humans. The aim of this investigation is to study the effect of administration of dimethylnitrosamine in mice fed a diet balanced in amino acids, or proteins enriched in sulfur containing amino acids. The dietary conditions used will include high-quality proteins at a 20% level and enrichment of the proteins with the sulfur containing amino acids methionine, cysteine. These are introduced to test the theory that a reaction of the sulfur groups with dimethylnitrosamine intermediates will prevent interaction with the susceptible DNA molecules. The objectve is to follow the protective influence such a diet has on the occurence of single strand breaks in DNA caused by dimethylnitrosamine. Methods. Quantitation of the breaks produced in DNA will be done by labeling with 125 iodine followed by separation of the denatured DNA on polyacrylamide gel electrophoresis and autoradiography. Labeling of DNA at the break points with gamma-32P-ATP and polynucleotide kinase (EC 2.7.1.78) will also be tested. DNA will be labeled at the free 5'OH group with 32P and after separation by polyacrylamide gel electrophoresis radioactivity determined. A further step is formation of initiation complex for RNA synthesis with Escherichia coli RNA polymerase containing sigma factor (EC.2.7.7.6) and gamma-32P-ATP or gamma-32P-GTP as the initiation nucleotide. These may be replaced by 14C or 3H labeled ATP or GTP. The heparin resistant complex is precipitated, excess of radioactivity removed and the remaining radioactivity determined. An unspecific initiation occurs in vitro at each single strand break.